github: Prevent duplicate workflow runs

The push and pull_request actions caused the workflows to be run twice
on PRs. Only run on push on the main branch (i.e. when the PR is merged)

Signed-off-by: Philip Molloy <philip@philipmolloy.com>

Author: pamolloy

.github/workflows/markdownlint.yml

@@ -1,6 +1,8 @@
 name: Markdown Lint Action
 on:
   push:
+    branches:
+      - main
   pull_request:
   workflow_dispatch:
 

.github/workflows/vale.yml

@@ -1,6 +1,8 @@
 name: Vale Action
 on:
   push:
+    branches:
+      - main
   pull_request:
   workflow_dispatch: